Mayor Walter E. Washington announced yesterday that the city is unilaterally taking a bigger share of the capacity of the regionally used Blue Plains sewage treatment plant, bringing the area-wide competition for new business and other tax-producing development to the brink of open war.

The mayor informed the other users of the gaint plant - Montgomery.Prince George's and Fairfax counties - that the District is increasing its present allocation of 135 million gallons daily to 140 million gallons.

The extra 5 million gallons - equivalent to about 16,500 new houses - would forestall a sewer moratorium in the city and permit it to maintain its present boom in new commercial and residential construction.

But it could also put a pinch on similar development in the suburbs especially in Montgomery County, which is already rationing sewer hookups.

". . . The District can no longer . . . underwrite the development of other jurisdictions by gambling with its own share of that growth," Washington said in a letter to the leaders of other Blue Plains users.

The action was immediately attacked by the other jurisdictions.

"Montgomery County does not find the unilateral reallocation of capacity acceptable," said David Sobers, director of environmental planning in that country.

"The action apparently violates court orders and agreements," said Fairfax Board Chairman John F. Henrity.

Even one of the District's biggest boosters, R. Robert Linowes, head of the Metropolitan Washington Board of Trade, was upset by the mayor's decision which city officials have been warning was coming.

"I don't think it will solve anything," Linowes said. "I think it will be fought by the other jurisdictions with all the means at their disposal . . . It would have been better to negotiate."

But in his letter. Washington said year of negotiating have gotten nowhere. "It appears we are still no further along [now] than we were in 1970 . . ." he said.

In justification of the District's action. Washington said the city's 135-million-gallon allocation under 1970 and 1974 regional agreements was "temporary or initial capacity."

The 1970 agreement assumed that a second regional plant would be built in suburban Maryland, and that when that happened. Montgomery and Prince George's could yeild some of their Blue Plains capacity to the District.

But those expectations were dashed when prince George's refused to permit a huge expansion of its plant at Piscataway and later when Montgomery failed to win federal approval for a big regional plant at Dickerson in the upper western part of the county.

Meanwhile, the District has recovered from the economic decline triggered by the 1968 riots and entered a period of accelerated and widespread development.

There are new office buildings downtown, new and rebuilt houses throughout the city and sheaves of plans for many other projects - redevelopment of Pennyslvania Avenue and construction of a convention center near Mount Vernon Square, among them.

All this development requires sewerage. But according to an analysis by the Metropolitan Washington Council of Governments, the district is about to use up its sewage allocation at Blue Plains. According to the COG study, the District would have a deficit at the plant of 2.2 million gallons daily by 1980.

However, while the District has been moving toward a deficit, studies commissioned by the D.C. Department of Environmental Services have shown that a large part of the Sewage the city sends to Blue Plains is actually water that is infilitrating from other source into leaky sewage pipes.

For example, 4 1/2 million gallons were reported leaking each day from the McMillan Reserovir. One District official estimated that of 11 million gallons daily of infiltration into residential Northwest, 9 1/2 million gallons - almost twice the amount the city took in additional capacity yesterday at Blue Plains - could be stopped on a cost effective basis.

In his letter, Mayor Washington said, "I do not believe that this action will cause any disruption to any other jurisdiction." He said that unused sewage treatment capacity at all plants in the area amount to 65 million gallons daily - enough to build the equivalent of more than 200,000 houses or about a decade's worth of regional residential construction.

But Montgomery's Sobers said it will not be easy to share the unused capacity because there are not enough connecting pipelines available.

Sobers said that if Montgomery had to absord half of the capacity loss created by the District's move yesterday, "that would be a severe blow to us . . . We (already) have a waiting list that amounts to 12 million gallons daily."

Montgomery hopes to build a 20-million-gallon-a-day plant at Potomac, but even if plans clear all hurdles, the facility probably would not be ready for four to five years.

While saying "my responsibility must be first and foremost to the people of the District of Columbia." Washington in his letter urged leaders of all the affected jurisdictions to "meet promptly" to discuss sewage problems.

Those problems have been aggravated - as Washington pointed out - by the federal government's decision not to fund any more sewer plant construction in the metropolitan area, saving it was up to the region to pay for its growth.